1. Field of the Invention
This invention relates to a method and apparatus for evaluating the strain in steel, silicon and other crystalline substances.
2. Description of the Prior Art
The strain in steel and other crystalline substances has conventionally been evaluated commonly by determining the spreading of diffracted beams of X-rays passed therethrough or determining their Vickers hardness.
But X-ray diffraction analysis is unfit for the evaluation of strain in a very small area because beams of X-rays are difficult to focus to such a small area as under 10 .mu.m across. Another shortcoming is low sensitivity of the spreading of diffracted X-ray beam to the strain. Evaluation by Vickers hardness, on the other hand, takes advantage of a phenomenon that the hardness of a material varies with its dislocation density (or strain). But this method also does not provide very accurate evaluation because solid solutions and precipitates in crystals also affect the hardness of the material being examined.
Electron Channeling Pattern (hereinafter abbreviated ECP) is a phenomenon discovered by D. G. Coates (D. G. Coates: Phil. Mag., 16 (1967), p. 1179). When electron beams are irradiated on a specimen of crystalline substance not too thin, part of the electrons having entered the specimen is elastically scattered while maintaining the incident energy because of the interaction with the constituent atoms of the crystal, with the rest being inelastically scattered losing the incident energy. Part of the incidence energy lost is used for the excitation of the electrons in the atoms making up the crystalline substance. Of the excited electrons, those emitted from the surface of the specimen are called secondary electrons. Of the inelastically scattered electrons, those emitted from the surface of the specimen are called back-scattered electrons. When the surface of the specimen is scanned with electron beams irradiated at varying angles, the intensity of the secondary and back-scattered electrons changes greatly because of the diffraction at the crystal plane in the vicinity of the Bragg angle .theta..sub.B at which the incident angle .theta. of the electron beams with respect to the crystal plane satisfies Bragg's law n.lambda.=2d sin .theta., wherein n is the order of reflection, .lambda. the wavelength of electron beams, and d the interval between crystal planes. On detecting the intensity of the secondary or back-scattered electrons and inputting the signal representing the detected intensity to a CRT display or other recording device synchronously with the scanning signal of the electron beams, an image with varying light and shade appears at and near the Bragg angle .theta..sub.B. Scanning electron microscopes are widely used to obtain ECP's.
The ECP is known to provide information about the orientation and perfectness of crystals. Regarding the perfectness of crystals, it is known that the ECP blurs when crystals are strained (D. E. Newbury and H. Yakowitz: Practical Scanning Electron Microscopy, ed, by J. I. Goldstein and H. Yakowitz (1975), p. 149 [Plenum Press]). But the intricateness of the ECP and other factors have so far hampered the quantitative evaluation of strains. Though it is known that strains can be quantified using the contrast of specific pseudo-Kikuchi lines in the ECP as a parameter (Wear, 1976, 40, p. 59), this method is inapplicable to the quantification of strains in an arbitrarily chosen crystal orientation.
Evaluation of the nonuniform distribution of work-induced strain, the amount of strain accumulated in different crystal orientations and the residual strain resulting from softening is extremely important for the development of recrystallization and texture control technologies. As such, there is a pressing need for the development of a reliable micro-strain evaluation method based on the conventionally established principles.
Noting a phenomenon that the ECP suddenly blurs when a micro-strain is applied on crystals, the inventors made an extensive analysis of ECP's using an image analyzer. The analysis led to a new discovery that image analysis of the ECP is very effective for the evaluation of the microstrain in such crystalline substances as steel.